As is known, vinyl aromatic polymers are thermoplastic resins suitable for being transformed, under heating, into shaped articles by injection or extrusion molding. Said vinyl aromatic polymers have a fair tensacity, but they are not suitable for use in applications where a high tenacity associated with good chemical resistance is required.
A way for improving those lacking properties is to provide a blend with other polymers having the deficient properties to dotain a material exhibiting the desired combination of properties. However, this approach was successful only in a few cases; generally, in fact, the blending results in the combination of the worst characteristics of each component, the overall result being a material of such poor properties as not to be of any practical or commercial value.
The reasons for this drawback are the fact that not all polymers are compatible with each other and, therefor, do not perfectly adhere. As a result, interfaces are formed among the components of the blend which represent weakness and breaking points.
More particularly, the blending of polystyrene, or generally of a vinyl aromatic polymer or copolymer, either as such or made impact resistant by grafting with a rubber, and a polyolefine, gives rise to blends endowed with heterogeneous structure and mechanical weakness, due to the incompatibility between these two types of polymers.
U.S. Pat. No. 4,386,187 discloses a thermoplastic composition comprising from 54 to 82% by weight of a polyolefin, from 15 to 40% by weight of a vinyl aromatic polymer and from 2 to 10% by weight of a styrene-butadiene-styrene thermoplastic block copolymer.
These compositions are homogenous and can be used for the production of shaped articles; however their poor mechanical characteristics in terms of modulus and IZO D resilience do not make them suitable to be used in the applications where these properties are required.
From published European Patent Application No. 291, 352, it is known that homogeneous polymeric compositions containing a polyolefine and a vinyl aromatic polymer may be prepared by using a vinyl aromatic polymer containing, as elastomeric component, small quantities of a vinyl aromatic monomer-conjugated diene linear block polymer, and, as a compatibilizing agent between the polyolefine and the vinyl-aromatic polymer, a vinyl aromatic monomer-conjugated diene star-block polymer.
Particularly, the above reported European Patent Application No. 291, 352 discloses a polymeric blend comprising:
from 40 to 98% by weight of a vinyl aromatic polymer containing dispersed therein, as an elastomeric component, from 0.5 to 5% by weight of a vinyl aromatic monomer-conjugated diene linear block copolymer; PA1 from 1 to 54% by weight of a polyolefine; and PA1 from 1 to 20% by weight of a vinyl aromatic monomer-conjugated diene star block copolymer; the sum of the three components being equal to 100%. PA1 from 10 to 90% by weight of a vinyl aromatic polymer (A) containing, as an elastomer component, from 0.5 to 5% by weight of a vinyl aromatic monomer conjugated diene linear block copolymer and from 5 to 15% of a diene rubber; PA1 from 5 to 50% by weight of a polyolefine (B); and PA1 from 5 to 40% by weight of a vinyl aromatic monomer-conjugated diene linear block polymer (C), having a diene amount comprised between 20 and 80% by weight; the sum of the three components (A), (B) and (C) being equal to 100. PA1 from 60 to 80% by weight of a vinyl aromatic polymer (A); PA1 from 10 to 30% by weight of a polyolefine (B); and PA1 from 5 to 20% by weight of a vinyl aromatic monomer-conjugated diene linear block polymer (C); the sum of components (A), (B), and (C) being equal to 100. PA1 S, S.sub.1, S.sub.2 are non-elastomeric polymer blocks of a vinyl aromatic monomer of the above formula (I), having the same or different molecular weight; and PA1 B, B.sub.1 and B.sub.2 are elastomeric polymer blocks based on a conjugated diene having the same or different molecular weight.
The above reported blends are homogeneous and suitable for being transformed into shaped articles by extrusion, injection molding and/or thermoforming; however, they show some drawbacks which limit the use thereof in such processes. The main drawback is that the processing scraps cannot be reused owing to a drastic lowering of the mechanical properties of the blend, particularly the resilience and the breack elongation. This undesired effect involves a waste or a not-noble use of the processing scraps, the amount of which may be even 50-60% of the virgin product.